a. Field of the Invention
The present invention relates to an electrophotographic element, and in particular to an electrophotographic element comprising, in successive layers: a substrate and a photosensitive layer including a charge transport material and a specific disazo pigment (which is employed as a charge generation material).
b. Description of the prior art
As light-sensitive materials which have long been utilized in electrophotographic elements, there are widely known inorganic light-sensitive materials such as amorphous selenium, selenium alloys, cadmium sulfide, zinc oxide, etc., poly-N-vinyl carbazole and its derivatives. Among them, the amorphous selenium and selenium alloys are universally put to practical use because of their exceedingly superior photoconductivity. However, the above recited materials are observed to be defective in the following points. That is, in the case of amorphous selenium, since its light sensitive wavelength region is limited to the blue region, it is scarcely sensitive to the red region. A plurality of methods have been proposed in order to widen its sensitivity to the long wave region. However, since the selection of the light sensitive wavelength region is subject to various limitations, the amorphous selenium is unable to display a sufficient sensitivity to the long wave light. In the case of utilizing zinc oxide or cadmium sulfide as a light-sensitive material, it is necessary to add various kinds of sensitizers in order that said zinc oxide or cadmium sulfide may serve for practical purposes since its own light sensitive wavelength region is limited, too.
While the poly-N-vinyl carbazole, universally known as organic photoconductive material, is abundant in superior abilities such as transparency, film forming ability, flexibility, hole transportability and the like, there can be observed deficiencies in that it is itself scarcely sensitive to 400 to 700 mm visible light wave lengths and so forth.
As a photosensitive element which has successfully overcome the above mentioned deficiencies, Japanese Patent Publication No. 10496/1975 discloses one utilizing a charge transfer complex consisting of poly-N-vinyl carbazole and 2,4,7-trinitrofluorenone.
In Japanese Patent Publication Nos. 5349/1970, 3168/1974, 14914/1975, 10982/1976, etc. are furthermore discussed photosensitive elements of the type which comprises, in successive layers; a charge generation layer formed of amorphous selenium or selenium alloy and a charge transport layer, each layer being allotted its own function.
Still further, the following different photosensitive elements of the type which comprises, in successive layers: a charge generation layer including a different kind of pigment and a charge transport layer, have hitherto been developed. U.S. Pat. No. 3,837,851 discloses a photosensitive element which comprises a charge generation layer and a charge transport layer including at least one tri-arylpyrazoline. In U.S. Pat. No. 3,850,630 is disclosed a photosensitive element which comprises a transparent charge transport layer and a charge generation layer including an indigo dye. U.S. Pat. No. 3,871,882 discloses a photosensitive element which comprises a perylene tetracarboxydiimide derivativecontaining charge generation layer and a charge transport layer including a condensate of 3-bromopyrene-formaldehyde resin. Some of the above mentioned photosensitive elements have already been commercially available. However, the fact is that there have not been developed yet photosensitive elements capable of satisfying various proposed properties in full degree.